Etch apparatus

ABSTRACT

An etch apparatus. The etch apparatus includes a tank coupled to a recirculating path that includes a dissolver. The dissolver includes a porous carbon matrix filter coated with silicon nitride. An etchant from the tank circulates through the recirculating path and performs a selective etching of a structure in the tank in contact with the etchant. The structure includes silicon nitride on a pad layer that includes silicon dioxide. The selective etching is characterized by the silicon nitride on the pad layer being selectively etched by the etchant relative to an etching by the etchant of the silicon dioxide. The etch apparatus further includes: means for dissolving the silicon nitride coated on the filter into the etchant at a controlled dissolution rate sufficient to cause the selective etching; and means for coating the silicon nitride onto the filter to facilitate the selective etching.

This application is a continuation application claiming priority to Ser.No. 10/760,896, filed Jan. 20, 2004; which is a divisional applicationof U.S. Pat. No. 6,699,400, issued Mar. 2, 2004.

FIELD OF THE INVENTION

The present invention relates, in general, to semiconductor devicefabrication and, more particularly, to etch processes used in thefabrication of semiconductor devices.

BACKGROUND OF THE INVENTION

The fabrication of semiconductor devices and/or integrated circuitsoften requires removing certain materials from a semiconductor waferwhile leaving other materials on the wafer. This can be accomplished ina selective etch process that uses an etchant having different etchrates with respect to different materials. To characterize the selectiveetch process, an etch selectivity is defined as the ratio of the etchrate of one material to the etch rate of another material. For example,an aqueous phosphoric acid solution having a concentration ofapproximately 85 percent heated to a temperature between 165 degreesCelsius (° C.) and 185° C. is routinely used for removing siliconnitride structures from a semiconductor wafer while leaving exposedsilicon dioxide structures on the wafer. At the temperature of 165° C.,the phosphoric acid solution etches silicon nitride at a rate ofapproximately 6 nanometers per minute and etches silicon dioxide at arate of no more than 0.25 nanometers per minute. The resulting etchselectivity is at least 24:1.

The etch selectivity of an etch process depends on the temperature,concentration, and composition of the etchant. Consequently, the etchselectivity usually changes as more wafers are processed in the etchant.For example, the nitride to oxide etch selectivity of the etch processusing the phosphoric acid etchant is approximately 24:1 when the etchantis fresh. After processing approximately 1000 wafers having siliconnitride thereon, the etch selectivity increases dramatically to 50:1 orgreater. This selectivity variation adversely affects the efficiency,reliability, and yield of the semiconductor device and/or integratedcircuit fabrication processes.

Accordingly, it would be advantageous to have an etch process that has astable etch selectivity and an apparatus for performing the etchprocess. It is desirable for the etch process to have a high etchselectivity. It would be of further advantage if the etch apparatus canbe adapted from existing etch apparatuses.

SUMMARY OF THE INVENTION

A general object of the present invention is to provide an efficient andreliable etch process and an apparatus for performing the etch process.It is a further object of the present invention for the etch process tobe capable of producing semiconductor devices and/or integrated circuitshaving high performance, high reliability, and high yield. Anotherobject of the present invention is to implement the etch process withmodifications to existing etch apparatuses.

These and other objects of the present invention are achieved byadjusting and controlling the composition of the etchant during the etchprocess. For example, a selective etch modifier can be introduced intothe etchant. The selective etch modifier alters the etch rates ofcertain materials but has no significant effect on the etch rates ofother materials, thereby modifying the etch selectivity of the etchprocess. By monitoring and controlling the concentration of the etchrate modifier in the etchant, a stable etch selectivity is maintainedduring the etch process. The etch rate modifier can be either aselective etch intensifier or a selective etch rate suppressor. Theselective etch intensifier selectively increases the etch rate ofcertain materials. On the other hand, the selective etch rate suppressorselective decreases the etch rate of certain materials.

In a preferred embodiment of the present invention, a hot phosphoricacid solution is used as the etchant for etching the silicon nitride ona semiconductor wafer. A recirculating path is established for the hotphosphoric acid etchant. A high surface area structure such as, forexample, a carbon matrix filter is coated with silicon nitride. Thecarbon matrix filter is installed in the recirculating path for theetchant. As the etchant in the recirculating path flows through thecarbon matrix filter, it dissolves the silicon nitride coated on thecarbon matrix filter. The dissolved silicon nitride significantlyreduces the etch rate of silicon dioxide on the semiconductor wafer. Theetch rate of the silicon nitride on the semiconductor wafer issubstantially unaffected by the presence of the silicon nitride in theetchant. Therefore, the silicon nitride dissolved in the hot phosphoricacid etchant functions as an etch rate modifier that enhances the etchselectivity of the etch process. More particularly, the dissolvedsilicon nitride functions as a selective etch rate suppressor thatsubstantially inhibits the etch of silicon dioxide on the semiconductorwafer. The concentration of silicon nitride in the etchant can bemonitored and adjusted to maintain a stable etch selectivity of the etchprocess.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an etch apparatus in accordance withthe present invention;

FIG. 2 is a flow chart schematically illustrating an etch process inaccordance with the present invention;

FIG. 3 is a schematic diagram of another etch apparatus in accordancewith the present invention; and

FIG. 4 is a schematic diagram of yet another etch apparatus inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention are described herein withreference to the figures. It should be noted that the figures are notnecessarily drawn to scale. It should also be noted that elements havingsimilar functions are labeled using the same reference numerals in thefigures.

FIG. 1 is a schematic diagram of an apparatus 10 used in a wet etchprocess in accordance with the present invention. Apparatus 10 is alsoreferred to as a wet etcher or simply an etcher. Etcher 10 includes atank 11 filled with an etchant 12. Tank 11 and etchant 12 form anetchant bath for etching an object, e.g., a semiconductor wafer 15,submerged in the etchant bath. Tank 11 filled with etchant 12 is alsoreferred to as a bath 11 of etchant 12. A heating element 16 such as,for example, a filament is immersed in etchant 12 for adjusting andmaintaining the temperature of etchant 12 during the etch process. Tank11 has a drain 18. In a semiconductor device fabrication process, oldand contaminated etchant is periodically removed from tank 11 throughdrain 18 and tank 11 is then filled with new etchant. A spout 19 isconnected to a source of deionized water (not shown) and provides tank11 with deionized water, thereby adjusting the concentration of etchant12. Etcher 10 also includes a chamber 21 attached to a sidewall 14 oftank 11. Chamber 21 has an outlet 22 at its bottom. A conduit 23 couplesoutlet 22 of chamber 21 to an inlet 24 of a filtering system 25. Anotherconduit 27 has a first end connected to an outlet 26 of filtering system25 and a second end mounted adjacent to tank 11. During an etch process,etchant 12 in tank 11 overflows sidewall 14 into chamber 21. Etchant 12in chamber 21 is pumped back to tank 11 through conduit 23, filteringsystem 25, and conduit 27. Therefore, chamber 21, conduit 23, filteringsystem 25, and conduit 27 form a recirculating path for etchant 12 intank 11. The second end of conduit 27 serves as an outlet 29 of therecirculating path. Because etchant 12 in tank 11 reaches chamber 21 byoverflowing sidewall 14, sidewall 14 is also referred to as an overflowsidewall and chamber 21 is also referred to as an overflow chamber or anoverflow compartment. Overflow chamber 21 communicates with tank 11through overflow sidewall 14.

In addition, etcher 10 includes a substance dissolving system 32 in therecirculating path for etchant 12. During an etch process, substancedissolving system 32 introduces a substance into etchant 12 to modifythe characteristics of etchant 12, thereby achieving a desired resultsuch as, for example, a high etch selectivity, a stable etchselectivity, a contamination free etch process, etc. The type andquantity of the substance introduced into etchant 12 depend on thecomposition of etchant 12 and the desired result. Substance dissolvingsystem 32 is installed between the two ends of conduit 27 and includes asplit valve 34, a dissolver 35, a bypass conduit 37, and a merge valve36. Split valve 34 has one inlet and two outlets. Merge valve 36 has twoinlets and one outlet. The inlet of split valve 34 is coupled to theoutlet 26 of filtering system 25 via a section of conduit 27. Dissolver35 has an inlet connected to the first outlet of split valve 34 and anoutlet connected the first inlet of merge valve 36. The second outlet ofsplit valve 34 is coupled to the second inlet of merge valve 36 viabypass conduit 37. The outlet of merge valve 36 is coupled to outlet 29of the recirculating path via another section of conduit 27. Split valve34 and merge valve 36 control the partition of etchant 12 flowingthrough dissolver 35 and through bypass conduit 37.

Preferably, dissolver 35 has a high surface area coated with thesubstance to be introduced into etchant 12 during the etch process. In apreferred embodiment, dissolver 35 is formed by depositing the substanceon a porous structure such as, for example, a carbon matrix filter. Whenetchant 12 in the recirculating path flows through dissolver 35, thesubstance deposited on the porous filter is dissolved in etchant 12.

The dissolution rate at which etchant 12 dissolves the substancedeposited on the porous filter can be controlled by adjusting thetemperature of etchant 12 flowing through the porous filter. Preferably,substance dissolving system 32 includes a temperature controller (notshown), e.g., a heating coil and a cooling coil, for adjusting thetemperature of etchant 12 flowing through the porous filter. Analternative method for controlling the dissolution rate is changing thesurface area of the porous filter exposed to etchant 12. This can beachieved by partially submerging the porous filter in etchant 12 flowingthrough substance dissolving system 32 and adjusting the extent to whichthe porous filter is submerged in etchant 12. The dissolution rate canalso be controlled by adjusting the rate at which etchant 12 flowsthrough the porous filter. The flow rate of etchant 12 through theporous filter can be controlled by adjusting split valve 34, merge valve36, and a pump (not shown) in the recirculating path. Further, etchant12 adjacent dissolver 35 may be saturated with the substance dissolvedfrom the surface of the porous filter. This may adversely affect theetch process. Therefore, substance dissolving system 32 preferablyincludes a flushing system (not shown) that can be periodically turnedon to flush dissolver 35.

FIG. 2 is a flow chart schematically illustrating an etch process 50 inaccordance with the present invention. By way of example, etch process50 is a wet etch process using etcher 10 of FIG. 1 for etching siliconnitride structures and/or polycrystalline silicon structures onsemiconductor wafer 15. In a semiconductor device fabrication process,silicon nitride structures are formed on semiconductor wafer 15.Typically, a pad layer of silicon dioxide is formed between the siliconnitride structures and the surface of semiconductor wafer 15 to relievethe tension on the wafer surface. Other structures such as, for example,polycrystalline silicon structures can also be formed on semiconductorwafer 15. Preferably, the silicon dioxide layer on semiconductor wafer15 remains in place after etch process 50 so that it can protect theunderlying films or retain a uniform thickness for consistentperformance of the semiconductor devices fabricated on semiconductorwafer 15. Therefore, etch process 50 preferably has a high and stableetch selectivity so that and the etch of the silicon dioxide pad layeron semiconductor wafer 15 is substantially inhibited.

Etchant 12 for etching silicon nitride and/or polycrystalline silicon onsemiconductor wafer 15 is preferably an aqueous solution of phosphoricacid having a concentration of approximately 85 percent and atemperature between approximately 165 degrees Celsius (° C.) andapproximately 185° C. Silicon nitride is deposited on a high surfacearea structure such as, for example, a carbon matrix filter, whichserves as dissolver 35 in substance dissolving system 32. Etchant 12dissolves the silicon nitride deposited on the carbon matrix filter asit flows through dissolver 35. In tank 11, the silicon nitride dissolvedin etchant 12 alters the composition and modifies the characteristics ofetchant 12. More particularly, the silicon nitride reacts with thephosphoric acid in etchant 12 in a chemical reaction:Si₃N₄+7H₃PO₄→2(NH₄)₂HPO₄+H2Si(PO₄)₂+HSi₂(PO₄)₃   (1)The silicon phosphate acid compounds formed in the reaction are notvolatile, so etchant 12 does not lose significant amounts of the siliconphosphate acid compounds through evaporation. However, the siliconphosphate acid compounds are unstable. They react with the water inetchant 12 as described in the following equations:H₂Si(PO₄)₂+2H2O→2H₃PO₄+SiO₂   (2)HSi₂(PO₄)₃+4H₂O→3H₃PO₄+2SiO₂   (3)Therefore, the series of chemical reactions described by equations (1),(2), and (3) can be described by the following equation:Si₃N₄+2H₃PO₄+6H₂O→2(NH₄)₂HPO₄+3SiO₂   (4)

The silicon dioxide formed in etchant 12 suppresses the etch of thesilicon dioxide on semiconductor wafer 15 and has no significant effecton the etch rate of silicon nitride and polycrystalline silicon. Theetch selectivity of etch process 50 is significantly increased.Therefore, the silicon nitride deposited on the carbon matrix filter insubstance dissolving system 32 functions as a selective etch ratesuppressor during etch process 50.

Etch process 50 starts with preparing an etchant bath (step 51) byfilling tank 11 in etcher 10 with etchant 12 so that etchant 12overflows sidewall 14 and spills into overflow chamber 21. Heatingelement 16 in tank 11 maintains etchant 12 at a desired temperature,e.g., approximately 165° C., at which temperature the phosphoric acidsolution loses its water component through evaporation. Spout 19continuously adds deionized water into tank 11 to make up the water lostthrough evaporation, thereby maintaining a substantially constantconcentration of etchant 12.

A pump (not shown) pumps etchant 12 in chamber 21 through filteringsystem 25 and substance dissolving system 32 to establish arecirculating path for etchant 12 (step 52). The pump also controls therecirculating rate of etchant 12. Filtering system 25 reconditionsetchant 12 throughout etch process 50 by filtering out contaminants thatmay be present in etchant 12.

When etchant 12 flows through dissolver 35 in substance dissolvingsystem 32, the silicon nitride deposited on the carbon matrix filter isgradually dissolved in etchant 12 and introduced into tank 11 throughoutlet 29 of the recirculating path (step 53). The silicon nitridedissolved in etchant 12 changes the characteristics of etchant 12. Moreparticularly, the silicon nitride functions as a selective etch ratesuppressor to enhance the etch selectivity of etchant 12.

The introduction of the silicon nitride into etchant 12 continues whilesemiconductor wafer 15 is submerged in tank 11 of etchant 12. Theconcentration of the silicon nitride selective etch rate suppressor inetchant 12 determines the etch selectivity of etch process 50.Preferably, the concentration of the selective etch rate suppressor issufficiently high to substantially quench or inhibit the etch of silicondioxide on semiconductor wafer 15. It should be noted that a very highsilicon nitride concentration in etchant 12 may produce too much silicondioxide in etchant 12, thereby causing an undesirable effect of silicondioxide precipitating on semiconductor wafer 15. Preferably, anequilibrium between the consumption and production of silicon dioxide inetchant 12 is maintained at an appropriate level to achieve an etchselectivity approaching infinity while substantially inhibiting anysilicon dioxide deposition on semiconductor wafer 15. A desiredequilibrium is achieved when the selective etch rate suppressorconcentration in etchant 12 is, by way of example, approximately 0.5milligram of silicon nitride per milliliter of the phosphoric acidsolution. At this concentration, the etch selectivity of etch process 50approaches infinity to one and there is no significant silicon dioxideprecipitation on semiconductor wafer 15 during etch process 50.

The concentration of the selective etch rate suppressor in etchant 12(step 54) is monitored. In one embodiment, the concentration of theselective etch rate suppressor is monitored by measuring the etch ratesof the silicon nitride structures and the silicon dioxide structures onmonitoring wafers (not shown) in etchant 12. In another embodiment, theconcentration of the selective etch rate suppressor is monitored bymeasuring the ammonium cation concentration in etchant 12. As describedin equations (1) and (4) above, the ammonium cation concentration in thephosphoric acid solution depends on the dissolved silicon nitrideconcentration in etchant 12. Methods for measuring the ammonium cationconcentration include cation ion chromatography and ammonia selectiveelectrode measurement.

Adjustments are made to etchant 12 if the monitoring scheme indicatesthat the concentration of the selective etch rate suppressor therein isnot optimal. If the concentration of the selective etch rate suppressorin etchant 12 is too low, the dissolution rate of the silicon nitridedeposited on the carbon matrix filter is increased. This can beaccomplished by increasing the temperature of etchant 12 flowing throughdissolver 35, increasing the surface area of dissolver 35 in etchant 12,and/or increasing the flow rate of etchant 12 through dissolve 35. Ifthe concentration of the selective etch rate suppressor in etchant 12 istoo high, the temperature and/or the flow rate of etchant 12 throughdissolver 35 are decreased to reduce the dissolution rate of the siliconnitride deposited on the carbon matrix filter in etchant 12. Thedissolution rate can also be reduced by decreasing the surface area ofdissolver 35 exposed to etchant 12 flowing through substance dissolvingsystem 32. The temperature of etchant 12 flowing through dissolver 35 isadjusted using a temperature adjusting element or an etchant temperaturecontroller (not shown), e.g., heating coil and a cooling coil, insubstance dissolving system 32. The flow rate of etchant 12 throughdissolver 35 can be controlled by adjusting the recirculating rate ofetchant 12. The flow rate of etchant 12 through dissolver 35 can also becontrolled by adjusting split valve 34 and merge valve 36 to alter theratio of etchant 12 flowing through dissolver 35 with respect to thatflowing through bypass conduit 37. Split valve 34 and merge valve 36 arepreferably capable of directing all etchant 12 in the recirculating paththrough dissolver 35, thereby maximizing the dissolution rate of siliconnitride into etchant 12. Likewise, split valve 34 and merge valve 36 arealso preferably capable of directing all etchant 12 flowing in therecirculating path through bypass conduit 37, thereby achieving asubstantially zero dissolution rate of silicon nitride into etchant 12.The dissolution rate of the silicon nitride deposited on dissolver 35into etchant 12 can also be adjusted by periodically flushing dissolver35 with deionized water.

After an appropriate silicon nitride concentration in etchant 12 isachieved, semiconductor wafer 15 is submerged in etchant 12 in tank 11(step 56). Usually, semiconductor wafer 15 is mounted on a cassette (notshown). The cassette includes a plurality of wafers mounted thereon. Thewafers mounted on a cassette are referred to as a batch of wafers. Byway of example, a batch typically includes between 15 and 20 wafers.Preferably, the wafers in a batch are substantially identical to eachother. In tank 11, the silicon nitride and/or polycrystalline siliconstructures on semiconductor wafer 15 are etched by the hot phosphoricacid. The etch of silicon dioxide on semiconductor wafer 15 is greatlysuppressed or substantially inhibited by the selective etch ratesuppressor in etchant 12.

When a desired etch result is achieved, etch process 50 ends by removingsemiconductor wafer 15 from tank 11 of etchant 12 (step 57). Preferably,steps 52, 53, and 54 described herein above and shown in the flow chartof FIG. 2 continue after semiconductor wafer 15 is removed from tank 11to maintain etchant 12 in tank 11 in an optimal condition. Etcher 10 isready for receiving the next batch of wafers. If etchant 12 is socontaminated that its continual use may adversely affect theperformance, reliability, or yield of the semiconductor devices onsemiconductor wafer 15, it is discharged from etcher 10 through drain 18at the bottom of tank 11. Tank 11 is then filled with new and cleanetchant 12. Filtering system 25 and dissolver 35 may also needreplacement from time to time. Further, the whole apparatus of etcher10, which includes tank 11, chamber 21, conduits 23 and 27, filteringsystem 25, and substance dissolving system 32, may need to be cleansedafter a prolonged use.

FIG. 3 is a schematic diagram of another etch apparatus 60 in accordancewith the present invention. Apparatus 60 is also referred to as a wetetcher or simply an etcher. Etcher 60 is structurally similar to etcher10 shown in FIG. 1 and includes a tank 11 filled with an etchant 12 anda deionized water supply spout 19. Etcher 60 also includes arecirculating path comprised of a chamber 61, a conduit 23, a filteringsystem 25, and a conduit 27.

A difference between etcher 10 of FIG. 1 and etcher 60 is that substancedissolving system 32 installed between filtering system 25 and outlet 29of the recirculating path of etcher 10 is absent in etcher 60. Instead,etcher 60 includes a dissolver 65 in chamber 61 adjacent to outlet 22.Like dissolver 35 in etcher 10, dissolver 65 preferably includes a highsurface area object coated with the substance to be introduced intoetchant 12. For example, when etcher 60 is used for etching siliconnitride on semiconductor wafer 15, dissolver 65 can include a carbonmatrix filter with silicon nitride deposited thereon. The siliconnitride dissolved in etchant 12 during an etch process functions as aselective etch rate suppressor to substantially inhibit the etch ofsilicon dioxide on semiconductor wafer 15. In an etch process usingetcher 60, dissolution rate of the silicon nitride on dissolver 65 iscontrolled by adjusting the temperature and the recirculating rate ofetchant 12.

Another difference between etcher 10 shown in FIG. 1 and etcher 60 isthat in etcher 60, chamber 61 is attached to a permeable sidewall 64 oftank 11. Etchant 12 in tank 11 flows into chamber 61 either throughpermeable sidewall 64 or by overflowing permeable sidewall 64. In otherwords, chamber 61 communicates with tank 11 through permeable sidewall64. Additional differences include the locations of deionized watersupply spout 19 and outlet 29 of the recirculating path. In etcher 60,deionized water supply spout 19 and outlet 29 of the recirculating pathare located in chamber 61. Therefore in etcher 60, recirculated etchant12 and deionized water are supplied to tank 11 via chamber 61 andthrough permeable sidewall 64 between chamber 61 and tank 11.

FIG. 4 is a schematic diagram of yet another etch apparatus 70 inaccordance with the present invention. Apparatus 70 is also referred toas a wet etcher or simply an etcher. Etcher 70 is structurally similarto etcher 10 shown in FIG. 1 and includes a tank 11 filled with anetchant 12 and a deionized water supply spout 19. Etcher 70 alsoincludes a recirculating path comprised of a chamber 71, a conduit 23, afiltering system 25, and a conduit 27.

A difference between etcher 10 of FIG. 1 and etcher 70 is that substancedissolving system 32 installed between filtering system 25 and outlet 29of the recirculating path of etcher 10 is absent in etcher 70. Instead,etcher 70 includes a substance dissolving system 72 installed betweenoutlet 22 of chamber 71 and inlet 24 of filtering system 25. Substancedissolving system 72 is comprised of a split valve 74, a dissolver 75,and a bypass conduit 77. Split valve 74 has one inlet and two outlets.The inlet of split valve 74 is coupled to the outlet 22 of chamber 71via a section of conduit 23. An inlet of dissolver 75 is connected tothe first outlet of split valve 74. Another section of conduit 23couples an outlet of dissolver 35 to inlet 24 of filtering system 25.Bypass conduit 77 is coupled between the second outlet of split valve 74and inlet 24 of filtering system 25. Split valve 74 controls thepartition of etchant 12 in the recirculating path flowing throughdissolver 75 and through bypass conduit 77.

Like dissolver 35 in etcher 10, dissolver 75 preferably includes a highsurface area structure coated with the substance to be introduced intoetchant 12 during the etch process. In a preferred embodiment, dissolver75 is formed by depositing the substance on a porous filter such as, forexample, a carbon matrix filter. When etchant 12 in the recirculatingpath flows through dissolver 75, the substance deposited on the porousfilter is dissolved in etchant 12.

The dissolution rate can be controlled by adjusting the temperature ofetchant 12 flowing through dissolver 75. Like substance dissolvingsystem 32 in etcher 10, substance dissolving system 72 preferablyincludes a temperature controller (not shown), e.g., a cooling coil, foradjusting the temperature of etchant 12 flowing through dissolver 75.The dissolution rate can also be controlled by adjusting the rate atwhich etchant 12 flows through dissolver 75. The flow rate of etchant 12through dissolver 75 can be controlled by adjusting split valve 74and/or a pump (not shown) in the recirculating path. Further, substancedissolving system 72 preferably includes a flushing system (not shown)that can be periodically turned on to flush etchant 12 near dissolver75.

Another difference between etcher 10 shown in FIG. 1 and etcher 70 isthat in etcher 70, chamber 71 is attached to a permeable sidewall 64 oftank 11. Etchant 12 in tank 11 flows into chamber 71 either throughpermeable sidewall 64 or by overflowing permeable sidewall 64. In otherwords, chamber 71 communicates with tank 11 through permeable sidewall64. Additional differences include the locations of deionized watersupply spout 19 and outlet 29 of the recirculating path. In etcher 70,deionized water supply spout 19 and outlet 29 of the recirculating pathare located in chamber 71. Therefore in etcher 70, recirculated etchant12 and deionized water are supplied to tank 11 via chamber 71 andthrough permeable sidewall 64 between chamber 71 and tank 11.

By now it should be appreciated that an etch process and an apparatusfor performing the etch process have been provided. In accordance withthe present invention, a selective etch rate suppressor is introducedinto the etchant bath during the etch process to increase the etchselectivity of the etch process. For example, in an etch process usinghot phosphoric acid etchant to etch silicon nitride on a semiconductorwafer, silicon nitride is introduced into the etchant as the selectiveetch rate suppressor. The silicon nitride in the phosphoric acid etchantsignificantly decreases the etch rate of silicon dioxide on thesemiconductor wafer. Preferably, the silicon nitride is introduced intothe etchant using a filter coated with the silicon nitride and installedin the recirculating path for the etchant. The silicon nitride isdissolved in the etchant while the etchant in the recirculating pathflows through the filter. When the etchant flows back to the etchantbath, the silicon nitride is substantially completely dissolved in theetchant, thereby substantially eliminating the particulate deposition ofthe silicon nitride on the semiconductor wafer. The silicon nitrideconcentration in the etchant is monitored and maintained at a desirablelevel by adjusting the temperature and flow rate of the etchant throughthe filter. The etch process of the present invention is efficient andreliable. The increased and stabilized etch selectivity improves theperformance, reliability, and yields of semiconductor devices and/orintegrated circuits fabricated using the etch process of the presentinvention.

While specific embodiments of the present invention have been shown anddescribed, further modifications and improvements will occur to thoseskilled in the art. For example, the selective etch rate suppressor isnot limited to being coated on a porous filter in the recirculating pathof the etchant and dissolved in the etchant as the etchant flows throughthe filter. The selective etch rate modifier can be introduced into theetchant in powder form. The powder can be either added directly into theetchant bath, or introduced into the etchant in the recirculating path.Further, the application of the present invention is not limited toenhancing the etch selectivity of an etch process. The principle of thepresent invention is applicable to other processes whose characteristicsare improved by introducing a material not required for the processitself. This process improvement is not limited to etch selectivityenhancement. For example, in a hydrofluoric acid based etch process foretching silicon dioxide on a semiconductor wafer, silicon can be coatedon a filter installed in the etchant recirculating path and introducedinto the etchant as the etchant flows through the recirculating path.The silicon serves to getter copper contamination. More particularly,the silicon removes the copper from the etchant, thereby avoiding thecopper being deposited on the exposed silicon on the semiconductor waferand contaminating the wafer surface.

1. An etch apparatus, comprising: an etching tank comprising an etchant,said etchant configured to perform a selective etching of a structure inthe tank in contact with the etchant, wherein the structure comprisessilicon nitride on a pad layer comprising silicon dioxide, wherein saidselective etching is characterized by the silicon nitride on the padlayer being selectively etched by the etchant relative to an etching bythe etchant of the silicon dioxide of the pad layer; a recirculatingpath coupled to the tank, wherein the recirculating path comprises adissolver, wherein the dissolver comprises a porous carbon matrix filtercoated with silicon nitride, and wherein the recirculating path isconfigured to receive the etchant from the tank and to circulate theetchant from the tank through the recirculating path, including throughthe dissolver, and back into the tank; means for dissolving the siliconnitride coated on the filter into the etchant at a controlleddissolution rate sufficient to cause the selective etching to decreasean etch rate of the silicon dioxide of the pad layer in the etchant in acontrolled manner such that an etch rate of silicon nitride of thesilicon nitride structure is substantially unchanged; and means forcoating the silicon nitride onto the filter to configure the coatedsilicon nitrate to facilitate said selective etching.
 2. The etchapparatus of claim 1, wherein the silicon nitride coated on the filterhas a different spatial distribution on the filter than would siliconnitride coated on the filter by being precipitated from the etchant ontothe filter while the etchant is passing through the filter.
 3. The etchapparatus of claim 1, wherein the tank comprises a heating element forheating and adjusting a temperature of the etchant, and wherein theheater is immersed in the etchant.
 4. The etch apparatus of claim 1,further comprising an etchant temperature controller adjacent saiddissolver, said etchant temperature controller adjusting a temperatureof the etchant flowing through said dissolver.
 5. The etch apparatus ofclaim 1, wherein the etchant comprises a phosphoric acid solution; 6.The etch apparatus of claim 1, wherein the recirculating path furthercomprises a chamber in contact with the tank, a filtering system, afirst conduit, and a second conduit comprising the dissolver, whereinthe chamber is in direct mechanical contact with the tank, wherein afirst end of the first conduit is connected to an outlet of the chamber,wherein an inlet of the filtering system is connected to a second end ofthe first conduit, wherein an outlet of the filtering system isconnected to a first end of the second conduit, wherein a second end ofthe second conduit is connected to the tank or the chamber, wherein therecirculating path is configured to receive the etchant from the tankinto the chamber and to flow the etchant from the chamber through thefirst conduit, the filtering system, the second conduit, and back intothe tank.
 7. The etch apparatus of claim 6, further comprising a spoutfor providing deionized water to the tank to adjust the concentration ofthe etchant in the tank, wherein the spout is directly connected to thechamber.
 8. The etch apparatus of claim 6, wherein a permeable sidewallis disposed between the tank and the chamber, and wherein the etchantflows from the tank to the chamber by flowing through the permeablesidewall and/or by overflowing the permeable sidewall.
 9. The etchapparatus of claim 8, wherein the etchant flows from the tank to thechamber by flowing through the permeable sidewall.
 10. The etchapparatus of claim 8, wherein the etchant flows from the tank to thechamber by overflowing the permeable sidewall.
 11. The etch apparatus ofclaim 6, wherein the second end of the second conduit is connected tothe chamber, and wherein the etch apparatus further includes: a splitvalve within the first conduit; and a bypass conduit external to thefirst conduit, wherein a first end of the bypass conduit is connected tothe split valve and a second end of the bypass conduit is connected to amerge location in the first conduit, wherein the dissolver is disposedwithin the first conduit between the split valve and the merge location,wherein the split valve is configured to split the etchant flowing intothe split valve from the first end of the first conduit into a firstetchant component flowing through the filter and a second etchantcomponent flowing through the bypass conduit, and wherein the secondetchant component merges with the first etchant at the merge locationinto a merged etchant flowing into the second end of the first conduitand into the inlet of the filtering system.
 12. The etch apparatus ofclaim 6, wherein the etch apparatus further includes: a split valvewithin the second conduit; a merge valve within the second conduit,wherein the dissolver is disposed within the second conduit between thesplit valve and the merge valve, wherein the merge valve is closer tothe second end of the second conduit than is the split valve, andwherein the second end of the second conduit is connected to the tank;and a bypass conduit connecting the split valve to the merge valve,wherein the bypass conduit is external to the second conduit, whereinthe split valve is configured to split the etchant flowing into thesplit valve from the first end of the second conduit into a firstetchant component flowing through the filter and a second etchantcomponent flowing through the bypass conduit, and wherein the mergevalve is configured to merge the first etchant component flowing throughthe filter and the second etchant component flowing through the bypassconduit into a merged etchant flowing into the second end of the secondconduit and into the tank.